In the manufacture of organic chemicals such as ethanol, n-butanol, n-propanol, acetone, and similar materials by fermentation one of the major problems is the recovery of the products from the fermentation liquor. The fermentation liquors contain the product in low concentrations-18% in the case of the yeast fermentation to ethanol, and 2.5% in the case of the Clostridium fermentation for n-butanol--and high energy costs are required to recover the materials in a pure state by conventional processes such as distillation. It is estimated that at least 70% of the energy content of ethanol is required to make it 99.5% pure. In the case of the n-butanol, with its much more dilute liquor, the energy load is much higher. There is current interest in using these alcohols made by the fermentation conversion of biomass materials as fuels. This is not feasible if the separation process uses the amount of energy required by conventional distillation procedures.
There have been other methods proposed for increasing the efficiency of the separation processes. One is by the use of freezing techniques wherein the water is frozen out of the alcohol in a multistage process. While this method is much more energy efficient than distillation, since the heat of fusion of water is substantially less than the heat of vaporization, the process is complicated by the requirements for separating the ice crystals from the liquor at each stage in the process. The capital costs for a freezing separation are substantially higher than for distillation. This essentially negates the thermal advantage.
The method which can be used to substantially improve the efficiency of separation is the use of solvent extraction. This requires the use of solvents with high relative solvency for the alcohol materials so that limited amounts of solvents will be used. In addition, it is necessary that the solvent can be removed from the alcohols with a minimum of energy use. In order to do this by distillation the heat of vaporization and the sensible heat required to evaporate the solvent should be low.
The instant invention uses fluorocarbon solvents which have very high solvency power for alcohols from the C.sub.2 to the C.sub.5 molecular size, have very low heats of vaporization, and which boil close to ambient temperature. Using these in conjunction with a solvent extraction apparatus as is shown in my co-pending application Ser. No. 06/076 250, that uses a combination of vacuum and pressure to effect the distillation near ambient temperature, will result in a low energy consumption separation of the alcohols and related compounds from the fermentation liquors.